A device of this kind is known from French Patent No. 2 513 862, which discloses a boot whose upper is composed of a sleeve and a rear cover, the lower part of the sleeve covering the shell base and carrying approximately radially to a hinge pin in the upper, an extension which, near its end, cooperates with a support point integrally joined to the shell base and defining a flection leaf spring which works counter to variations of the angle of forward lean of the upper.
According to this same patent, the active length of the leaf can be modified by adjustment means constituted by a slide which travels in a slide track and acts as a stop motion device on the extension forming the spring leaf, for which it determines the length of active flection.
The presence of the flection leaf has the disadvantage of constituting a weak point, since it is designed to oppose resistance in a bracing arrangement under the pressure exerted by the upper, this action becoming stronger as the distance separating the point of support from the hinge pin increases.
This leads to the use of high-performance, rupture-resistant, and therefore expensive materials.
European Patent No. 0 172 159 discloses the formation of a transverse groove in the sleeve or front cover of the upper. This groove houses an elastic support element whose elastic properties can be modified in localized fashion by a rigid, movable cursor which is fastened to it and which can be moved along the elastic element in the groove in order to modify its resistance to compression.
This configuration has the disadvantage of complicating the shape of the front cover and the technique used for its construction. Moreover, because the transverse groove extends on either side of the area corresponding to the instep/flection fold, this flection-control device cannot be applied to boots whose upper opens on the front.
In addition, German Patent No. 35 30 243 discloses a flection-control device also involving the use of a rigid movable cursor whose active portion slides on the lower edge of the sleeve or front cover of the upper so as to be stopped, during flection, on a part of the shell base. The active portion of the cursor is extended by a guidance slide arranged as a stop motion device between the lower edge of the sleeve and a groove substantially parallel thereto.
In this type of construction of a flection-control device, manufacture of the shape of the front cover is relatively complex. Moreover, in the event of deterioration of the lower edge of the sleeve, the entire upper must be detached in order to repair it.